Author

Abstract

The recognition of fault-perturbed stress fields is an important tool in areas of mineral or hydrocarbon exploration. The Bowie underground coal mines of western Colorado expose a reverse-reactivated growth fault that perturbed the stress field during cleat (fracture) formation, rotating cleat orientations up to 500 m on both sides of the fault. Two unusual fracture types are found only in coal adjacent to the fault: (1) concentric cleats, highly curved fractures that form blocks resembling balls or eggs and (2) horsetail cleats, striated surfaces that superficially resemble shattercones and result from shear failure in coal. Numerical models created with the boundary element program Poly3D were used to estimate the magnitudes and orientations of the paleostress axes during cleat formation, taking into account the depth of burial, 3-D fault orientation, elastic rock parameters, and far-field stress states. When the elastic rock parameters and modeled orientations of the stress axes are held constant, the relative stress ratio, R = (σ_1 — σ_2)/(σ_2 — σ_3), uniquely determines the orientations of fractures forming in the fault-perturbed stress field. Comparison of the models with systematic observations on both sides of the fault allows the selection of a best-fit model. If the depth of overburden during fracture formation is known, this technique can be used to estimate the magnitude of σ_1 in fault-perturbed areas. The rotated face cleats and unusual, fault-related cleat types provide unequivocal evidence of a fault when (1) the fault predates cleat formation and (2) the far-field horizontal stress during cleat formation is oblique to fault strike. In addition, the varying spatial association of these fault-perturbed cleat styles with the fault may provide a qualitative estimate of fault location while mining. Pre-existing faults also strongly control reactivation-related folding, which at formed several low-amplitude folds, including a footwall fold. Igneous sills in three Bowie coal seams show strong preference for fault zones. The recognition of similar fracture trends in other mining or exploration areas is a valuable tool that may significantly reduce economic or human cost by helping to mitigate fault-related hazards and highlighting potentially productive zones in faulted reservoirs.